GroundwaterRisks GlobalStabilityCheck RetainingWallFailure

The Slope That Looked Safe: Anatomy of a Retaining Wall Failure

*Failure of a retaining wall in a rather ''gentle'' sloping profile. Source: Structures Magazine*

In a quiet riverside development, a seemingly routine retaining wall project turned into a landslide. A 10-meter-high gentle slope, supporting a high-rise tower and light ancillary facilities, suddenly gave way mid-construction. Soil surged into the adjacent river, disrupting flow, damaging a pedestrian path, and causing minor injuries. Fortunately, the tower stood firm. The retaining wall, however, collapsed spectacularly.

This was no steep cliff or perilous terrain—it was a gentle slope. But beneath the surface lay a quiet villain: a soft soil layer that escaped detection during the initial borehole investigations. Those boreholes were concentrated at the tower’s footprint, not the slope or wall location. The result? An incomplete ground profile and a devastating misjudgment.

Missed Red Flags and the Global Stability Blind Spot

The engineer followed conventional practice: L-shaped reinforced concrete design, backfill resisting sliding and overturning. What wasn’t checked? Global stability—the deep-seated, overall safety of the entire slope-structure system. The designer assumed the slope’s geometry and stiff soil at the tower base meant safety. But the soft soil layer extended toward the wall’s base, creating an unstable slip plane beneath.

Codes like Eurocode 7 and the Canadian Foundation Manual explicitly recommend checking overall stability—especially when cohesive soils or slopes are involved. A gentle appearance doesn’t mean gentle mechanics. Soil can shift, especially when water enters the equation.

And water did come. Rainfall, seepage, and possibly perched water tables increased pore pressures, reduced shear strength, and induced soil instability.

The default soil model is depicted including clay and water regimes. Source: Structures Magazine

Groundwater, Soft Soils, and the Art of Seeing the Unseen

Rain is a slope’s worst enemy. Water doesn’t just saturate—it destabilizes. It adds weight, weakens cohesion, and builds internal pore pressure that reduces effective stress. When soft soils lie near the surface, especially unconfined at a slope’s toe, the threat multiplies.

Finite element modeling later confirmed what hindsight already knew: with water and soft clay included, the slope’s factor of safety dropped to a worrying 1.35. That’s far from the 1.5 minimum many codes demand. The collapse could have been anticipated—and likely avoided upon detailed investigation and analysis.

Final Thoughts

This incident wasn’t a failure of materials or luck—it was a failure of assumptions. Site investigation must extend to every area of influence, not just the tower. Desk studies must precede construction works. Global stability checks must be standard practice, not optional extras. And the hidden threat of groundwater must always be accounted for, especially in the era of climate-driven rainfall extremes.

Let this gentle slope be a loud wake-up call for engineers everywhere.

Source: structuremag.org